Attenuation of the midbrain-evoked defense reaction by selective stimulation of medullary raphe neurons in rats.

In rats anesthetized with alfaxalone/alfadolone, electrical stimulation in the dorsal periaqueductal gray matter (PAG) evoked a cardiovascular defense reaction with increases in blood pressure, heart rate, femoral conductance, and respiratory activity but a decrease in renal conductance. These responses were attenuated significantly after microinjection of 200 nl 0.1 M DL-homocysteic acid (DLH) into nucleus raphe magnus (NRM, n = 12) or nucleus raphe obscurus (NRO, n = 22). The maximum reduction in the pressor response was similar after stimulation in NRM (-36.3%) and NRO (-37.3%). The reduction of the tachycardia, however, was greater after stimulation in NRM (-78.1%) compared with NRO (-34.6%). The maximum reduction in the PAG-evoked vasodilation of the hindlimb averaged -55.7% and -55.3% after stimulation of NRM and NRO, respectively, whereas the renal vasoconstriction was reduced by -66.4% and -79.0%. The PAG-evoked increase in respiratory amplitude was attenuated only after stimulation of NRO. It is concluded that neurons in NRM and NRO may be involved in modulating the level of excitability of neurons in the midbrain defense area and/or in its efferent pathway.